usnic: make this show_help message like the others

There's no need for the port number (since usNIC has no port numbers),
and make the wording the same as other help messages.

Reviewed by Reese Faucette.

cmr=v1.8.2:reviewer=ompi-rm1.8
usnic: EHOSTUNREACH means there is no route

ibv_create_ah() can also return EHOSTUNREACH, which means that there
is no route to the peer.  Treat that as a non-fatal warning.

Reviewed by Reese Faucette.

cmr=v1.8.2:reviewer=ompi-rm1.8
Silence warning